Institute for energy technology in Norway (IFE) has, since the nineteen sixties, worked with development of tracer technology for industrial applications. Since the beginning of the nineteen eighties the focus has been on the oil and gas industry. Many passive inter-well (well-to-well) tracers have been tested and qualified, and in recent years, some families of partitioning tracers have also been tested in laboratory and field experiments. The laboratory tests include flooding experiments at simulated reservoir conditions using sand-packed columns containing crude oil at residual oil saturation. The tracer candidates are also tested for thermal stability, and adsorption in closed vials with anaerobe atmosphere, with and without rock materials present.
Reactive partitioning tracers are injected as a pulse in a single well chemical tracer test (SWCTT). Due to the solubility of the reactive partitioning tracers in the oil phase, these tracers will move more slowly through the reservoir than the non-retained compounds. The reactive partitioning tracers are transported normally 2 to 6 meters (up to 15 meters) from the well bore by an additional injection fluid (cf. Tomich et al6; Deans and Majoros7). The well is then shut-in for a period of time, normally 1-10 days (Deans and Majoros7). During the shut-in an amount of the reactive partitioning tracer, reacts to form a new secondary tracer (generated in-situ reaction product) with a lower solubility in the oil phase, ideally significantly lower or “passive” (i.e. soluble only in the water phase). The well is then back-produced and the produced fluid is monitored for both the unreacted portion of the reactive partitioning tracer and the newly formed tracer. When the oil/water partition coefficient for the reactive partitioning tracer and the in-situ generated reaction product are known, the residual oil saturation can be calculated when the difference in migration times for the in-situ generated reaction product and the partitioning tracers have been measured. This concept was first described by Deans, H. A1 (U.S. Pat. No. 3,623,842). In addition, two other passive (not oil/water partitioning tracer) tracers have been used. The first is added to the whole injection volume and is used to monitor mass balance during the whole single well test. The second is added in the same volume as the reactive partitioning tracer and can be used as a back-up in case the reactive partitioning tracer has fully reacted during the shut-in. This tracer can only be used as a back-up for the reactive partitioning tracer if the reservoir drift during the shut-in is minimal.
The single well chemical tracer test (SWCTT) technology is a standard method for the determination of oil saturation in the near-well zone, and for evaluation of performance of Enhanced Oil Recovery (EOR) operations and EOR pilots. In conventional SWCTT, a tracer (e.g. ethyl acetate or propyl formate) with a known oil/water partition coefficient is introduced with the injection water. The oil/water partition coefficient of the tracer is determined by laboratory experiments. The well is then shut-in for a period of time and then back produced; samples of water are collected for analysis during this back production. During the shut-in some of the tracer reacts to form a secondary tracer in-situ, normally with a lower oil/water partition coefficient. In the case of ethyl acetate, two compounds are formed by hydrolysis ethanol and acetic acid. Normally ethanol is used as the secondary tracer, it can be analysed with the same analytical equipment as ethyl acetate. When the well is back produced, the tracers will move through the reservoir at different velocities based on the partition coefficients and the oil saturation in the volume surrounding the well. The oil saturation for a field with negligible oil flow rates compared to the water flow rates (a field close to residual oil saturation) can be described by chromatographic theory and calculated from the following equation:
                    S        =                                            T              1                        -                          T              2                                                          T              1                        -                          T              2                        -                                          T                1                            ⁢                              K                2                                      +                                          T                2                            ⁢                              K                1                                                                        (                  equation          ⁢                                          ⁢          1                )            
Here T1 and T2 are the retention times of the reactive partitioning tracer (eg ethyl acetate) and the secondary tracer (eg ethanol), respectively, S is the residual oil saturation, and K1 and K2 are the partition coefficient of the reactive partitioning tracer (eg ethyl acetate) and the secondary tracer (reaction product—eg ethanol), respectively,
If the partition coefficient is known, the residual oil saturation can be calculated from the measured difference in the arrival times between a non-partitioning/lower partitioning secondary tracer (the reaction product) and the reactive partitioning tracer. This equation is only valid as long as the tracers do not interact with the rock material.
Partitioning tracers are also used in inter-well tracer experiments, where typically no reaction takes place but the tracer is injected at one point and detected in the fluid produced at a second point. Different groups of chemicals have been tested for application as partitioning tracers in these methods. Important parameters are the partition coefficient, hydrolysis rate, the absence of adsorption to rock materials, and the analytical detectability.
Certain compounds, such as esters (eg ethyl acetate and propyl formate), have been used as reactive partitioning tracers in SWCTT, however due to poor limits of detection on-site and presences of the ethanol and methanol in the reservoir, large amounts of these tracers are needed to conduct a test. The reactive partitioning tracer is injected in a high concentration (around 1%), which is a drawback of the current system for several reasons. Firstly, this high concentration of tracer compound may begin to affect the system (oil saturation, water chemistry) which is to be investigated by the test. In addition, this limits the number of reactive partitioning tracers that can be used in a test, normally to one since the total concentration of tracer would otherwise be excessive. It would be advantageous to use several reactive partitioning tracers in the same test to get multiple results that could then be validated and inter-calibrated against each other.
Furthermore, compounds like ethyl acetate have low flash points (−4° C. for pure ethyl acetate). This makes handling and transportation of these compounds difficult and potentially dangerous. Health and safety is an issue of paramount importance when planning a SWCTT operation.
It is therefore of paramount importance to find new SWCTT tracers that can be used by injection in much lower concentrations than 1%, to reduced or remove their effect on the system. Such a large reduction in injection concentration would also allow the use of multiple tracers (if multiple SWCTT tracers are available) and thus allow multiple (eg 2-5) independent oil saturation measurements in a single SWCTT, improving confidence in the measurement. Health and safety is of cause of critical importance, finding new reactive partitioning tracers which can be detected in lower levels on-site would reduce the risk of (amongst other things) fire and chemical spillage compared to a conventional ethyl acetate SWCTT.
In order to be effective as a reactive partitioning tracer, a compound must display certain key properties for effective function. In particular, an effective partitioning tracer should display an appropriate, relatively stable, partition coefficient, an appropriate hydrolysis rate at the pH and temperature conditions of the reservoir, it should be acceptable from a health and safety point of view, it should not interact with rock and other material of the oil well and oil field, and it should be detectable at low levels. Suitable compounds would also advantageously be distinct from the compounds found naturally in oil reservoirs, such that injected compounds or their reaction products can be identified as such down to a low level.
The SWCTT method requires the tracers used to have several parameters within optimal ranges. Firstly the reactive partitioning tracer must have oil/water partitioning and not have any significant interactions with the rock material. Their oil/water partitioning characteristic must also be stable over a range of pH, the pH in the area surrounding the well may change during a SWCTT (eg range pH 5 to 8) due to the injection fluids used and the formation type etc. In addition, it should be detectable in low concentrations, preferably in the field. Secondly, it should undergo a reaction (change) during the shut-in period which produces one or more secondary tracers with several parameters within optimal ranges. The secondary tracer should have a significantly lower oil/water partitioning than the parent reactive partitioning tracer and not have any significant interactions with the rock material. It should be detectable in low concentrations, preferably in the field.
The art of reactive partitioning tracers has developed little over the past 40 years, with ethyl acetate still used as the reactive partitioning tracer of choice in SWCTT methods in spite of several disadvantages with this compound as discussed herein. The present inventors have undertaken to establish an alternative group of reactive partitioning tracers to address some or all of these issues after such a long period. It is not trivial, however, to find suitable new reactive partitioning tracers. Many compounds have been tested but few have been found effective. For example, several apparently likely prospects have shown a pH dependant oil/water partitioning coefficient, which would reduce or remove the information which could be derived from their use. Examples of compounds found not to be suitable for SWCTT reactive partitioning tracers are shown in Table 3.
There is evidently a considerable need for a new class of reactive partitioning tracers which can be used in SWCTT methods. Such tracers would advantageously not be naturally present in the fluid produced from oil reservoirs, would have a moderate partition coefficient which was little dependent upon pH, would partially react under the conditions of a well over several days to produce one or more secondary tracers with lower partition coefficients, would not degrade under the conditions of the well, would not have affinity for the rock or other structures of the well, and/or would be detectable at low levels, preferably with equipment useable at the production site.